One example of a transponder which relays an optical signal is disclosed in Literature 1 (Japanese Patent Application No. 2002-167726). Shown in Literature 1 as an example of a transponder is a transponder in which optical energy of an infrared ray incident on a light receptive element 11 is converted into electric energy to drive each electric circuit, and a central processing unit 13 reads response information stored in a storage unit 14, based on which response information, a signal light as a response signal is emitted from a light emitting element 16.
Since in the transponder recited in Literature 1 or the like, a bit rate of a client signal to be received will be 9.95328 Gbps in a case of an STM-64/OS-192 and will be 10.3125 Gbps in a case of a 10 Gbit Ethernet (registered trademark) to differ in an operating rate, manual switching of a different set frequency of a 10 Gbit signal having a different rate is required.